。
Keep the room clean.
Literal
Room [object-を] sanitary [make-にして] [in advance-ておき] [imperative-なさい].
Stacks three grammar patterns: ~にする (make into a state), ~ておく (do in advance/maintain), and ~なさい (polite command). 清潔 is a na-adjective meaning sanitary or hygienic — more formal than きれい. The combination にしておく conveys 'maintain in a state.'